This invention relates to transmission of digital signals by carrier waves, for example between a spacecraft and a ground station.
Whenever data are transmitted from a spacecraft to a ground station, the information to be transmitted consists of binary data resulting from sampling onboard the spacecraft and binary encoding. The resuling code, which is inappropriately called pulse code modulation (PCM) may be further encoded for purposes of error detection and correction, for instance by convolutional coding or Reed-Solomon coding. The resulting codes are converted into binary code or "non return to zero" (NRZ) format, or into split phase level (SP-L) format, which, advantageously, is a self-clocking code. The resulting baseband signals are used to perform a phase-modulation process of a carrier wave (PSK=phase shift keying). In this modulation process, a code transition translates into a phase inversion of the carrier or of the subcarrier wave. In the case of subcarrier modulation, the subcarrier frequency is coherent with the bit rate. Where the subcarrier has a positive zero crossing at the start of each bit, as prescribed by certain modulation standards, this feature may be used for resolution of the PSK-phase ambiguity.
A second modulation process takes place with relation to the PSK wave and which, according to normal practice, is a conventional phase modulation process. Whenever the first phase modulation process by baseband signals was performed on the carrier, this second modulation process is omitted, and the resulting modulation is known as binary phase shift keying (BPSK). Finally, the modulated radio frequency carrier wave may be up-converted and is transmitted to earth.
FIG. 1 is a functional block diagram of a conventional receiver of phase-modulated waves comprising a subcarrier. Basically, the inverse processes of modulation take place. A demodulator (1) phase-demodulates the received signals to produce the subcarrier modulated by the baseband. To warrant phase-coherent reception by the carrier, the demodulator (1) should be capable of determining or estimating the phase and frequency of the received signal with as little error as possible. This can be achieved by using a reference signal which is derived from the received wave's remanent carrier frequency component. This component can be tracked with a carrier-tracking loop (2). The subcarrier modulated by the baseband is passed on to a second demodulator (3) which then performs phase-demodulation to produce the baseband signal. Similar to the processing in the first demodulator, this demodulator (3) comprises a means of subcarrier tracking. Since PSK modulation does not leave a usable subcarrier component, the subcarrier tracking loop (4) must be a squaring loop or a Costas loop.
In the case of BPSK modulation, the carrier tracking loop 2 must be based on the same squaring or Costas principle, or an incoherent reception system may be used. In this case, subcarrier demodulation is omitted.
The baseband signals are subsequently passed onto a bit conditioner (5) which regenerates the bit clock signal and thus produces the bit stream and information on the confidence level if error correction and detection coding is used at the output. If a convolutional decoder is subsequently used, this is called "soft decision" and is coded in two bits. Finally, a decoder (6) reconstitutes the initial binary data by performing an appropriate decoding process, such as convolutional decoding followed by Reed-Solomon decoding.
FIG. 2 is a diagram of a phase demodulator which uses a Costas loop. The wave to be demodulated is passed on to two arms: a "data" arm and an "error" arm, each of which comprises a mixer or multiplier (7 and 8, respectively) and a matched filter (9 and 11, respectively). The signals provided by these two arms are multiplied in a multiplier (12), the output signal of which is filtered in a second order loop filter (13), wherein the filtered signal forms the frequency control signal of a variable oscillator (14) which supplies a reference frequency to the two multipliers (7 and 8), wherein the reference frequency supplied to multiplier 8 has a phase shift of 90.degree. for the "error" arm. In the case of the subcarrier tracking loop (4) of FIG. 1, for example, the two "data" and "error" arms generate baseband frequency signals and the central arm formed of elements 13 and 14 restitutes signals at the subcarrier wave frequency.
A demodulator apparatus as illustrated in FIG. 1 will exhibit degradation of information with respect to its theoretical characteristics: for a given probability of errors, a higher ratio of power to noise is required, where power is the signal power received and noise is the spectral density of the (white or Gaussian) noise received. In addition, this apparatus will exhibit threshold effects in the phase locked loops, which will cause the degradation to be higher nearer the threshold.
The need to reduce the degradation and lower the thresholds is enhanced when error detection and correction coding is used, since the very purpose of such coding is to reduce the energy per bit required on reception, that is, to reduce the transmit power or antenna size. In addition, the use of these codes entails an increase in channel width, that is, an increase of the bit rate. This results in a reduction of energy per bit to be demodulated. Finally, certain error detection and correction decoders, especially convolutional decoders, are designed on the assumption that the errors in the demodulated code occur randomly, that is, they follow Poissons distribution. However, the increased probability of errors which is the consequence of degradation is not time independent but presents itself in bursts of errors. The benefits of error detection and correction coding can be severely reduced by this effect.
Reasons for the occurrence of degradation are related to technological implementation of the receivers. Thus, for example, the use of filters in the receiver front end to improve the dynamic response of the system may impair the matched filter characteristics required and create group delay variations over the spectrum, causing more degradation and higher thresholds. Local oscillator jitter may require the use of phase locked loops having a bandwidth so large that data are tracked; alternatively, the resulting jitter on the reference signal degrades the data. When a Costas loop is used, usually the data filters in both arms are not matched but are only approximations of the data spectrum. This incrneases subcarrier jitter and makes the acquisition threshold higher.
Another disadvantage results from the fact that methods of subcarrier tracking from a biphase modulated signal exhibit a 180 degree phase ambiguity which results in a sign ambiguity of the baseband signal. Using the definition that the subcarrier has a positive zero crossing at the start of the bit can resolve this ambiguity. However, this can not be achieved with separate subcarrier demodulators and bit conditioners.
Finally, currently available demodulator devices comprise many components which are in analog technology such as operational amplifiers. Such analog components require regular fine tuning due to the occurrence of drift. Fine tuning can only be performed by highly skilled operators using specialized equipment and, in practice, the end result is more degradation and worse thresholds.
This invention pertains to a method for demodulating a carrier wave which is phase modulated by a subcarrier wave which is phase shift key modulated by baseband signals, according to which at least one phase demodulation is performed using a Costas tracking loop to regenerate the modulated wave, wherein the resulting baseband signals are subjected to a sampling and filtering process followed by a decoding process, a method which does not exhibit the aforementioned disadvantages.
In accordance with the method according to this invention, the frequency is first down-converted to an intermediate frequency which is lower than the frequency of the carrier wave, the subcarrier wave is then extracted by phase demodulation wave using a modified Costas loop, part of which is a digital circuit, and in which the subcarrier is regenerated by a waveform synthesizer and which comprises matched digital filters; the signal demodulated in the Costas loop is then digitally sampled at a freuency which is twice the intermediate frequency and the bit clock frequency is then regenerated in a first order phase locked loop, using the subcarrier wave frequency as a reference frequency.
The invention consists in using an intermediate frequency, of inverting the sequence of carrier and subcarrier wave demodulation in the Costas loop and replacing demodulation of the carrier by digital sampling of the demodulated signal in the Costas loop.
The frequency conversion to an intermediate frequency improves demodulator performance because the signal to noise ratio is better at intermediate frequency. This provides a higher signal component for carrier demodulation. The use of matched digital filters in the two arms of the Costas loop improves the subcarrier acquisition threshold since it allows for drift-free filtering and the use of phase locked loop filters with very narrow bandwidths. The interface between the analog and the digital signals is easily implemented, for instance by using integrated circuits of the VLSI type having a very high level of integration.
Use of a first order phase locked loop for bit clock regeneration controlled by the subcarrier wave is based on the fact that the subcarrier frequency and the bit clock frequency are coherent. Because of this, the bandwidth of the loop can be indefinitely reduced, which improves bit clock hold-in periods.
According to another characteristic of the invention, the bit clock frequency is obtained by determining its phase in a first order phase locked loop which multiplies a matched digital signal at the start of each bit with a matched digital signal at midbit, where the subcarrier frequency is divided by the resulting signal to produce the bit clock frequency.
Determination of the bit clock frequency phase provides information on the occurrence of bit slip, obviating the need to resort to the search process known as "node-switching."
According to yet another characteristic of the invention, the level of the Costas loop input signal is regulated according to the level of the digital output signal.
This regulation allows for adjustment of the input level of the Costas loop so as to compensate for the modulation factor.
According to yet another characteristic of the invention, the form of the reference waves generated by the waveform synthesizer is matched to the input signal waveform after it has been modified by the characteristics of the circuit which performs demodulation. In this way, the two waveforms processed by the demodulator circuits are matched to the maximum, which further improves the data signal to noise ratio.
The object of the invention is also to provide an apparatus for implementing the aforementioned method. In this apparatus, each external arm ("data" and "error" arms) of the Costas loop, sequentially from the modulated signal input, comprises a balanced demodulator mixer circuit which receives a waveform at the subcarrier wave frequency, a bandpass filter, a sampling multiplier which receives a signal at twice the frequency of the intermediate frequency, an analog to digital converter and a matched digital filter controlled by the bit clock frequency, the output of which is passed onto a multiplier which multiplies the "data" and "error" signals.
According to another characteristic of the apparatus according to the invention, the central branch of the Costas loop comprises, sequentially from the "data" and "error" multiplier, a second order loop filter, a variable frequency oscillator and a waveform synthesizer driven by the bit clock frequency and connected to the two demodulator mixer circuits by an analog to digital converter. The use of a digital waveform synthesizer more particularly allows production of all of the desired waveforms.
The apparatus according to the invention may also be employed for demodulation of signals modulated by binary phase shift keying (BPSK). To perform this demodulation process, subcarrier demodulation is omitted and the signal is digitally sampled at a frequency of four times the intermediate frequency.
Other characteristics and advantages of the invention will become apparent from the description below which is provided as an illustration of the invention and which refers to the drawings, wherein: